The present invention relates to a flexible surface heating element, particularly for seat heaters, comprising a heating field of conductive fibers electrically connected to at least one contact strip.
Furthermore, the present invention relates to a method for producing a flexible surface heating element, particularly a flexible surface heating element for seat heaters, comprising a heating field of conductive fibers, particularly carbon fibers, electrically connected to at least one contact strip.
It is common practice to heat seats in automotive vehicles electrically. This is generally realized by the measure that a heating interlayer is introduced between the seat core, which consists of foam or fiber material, and the cover. This heating interlayer is connected via a cable connection to the on-board electrical system of the automotive vehicle. An electrical conductor is heated by the power flow and outputs heat to the surface of the seat. Temperature regulation or temperature control is normally performed by means of electronic control devices that either sense the temperature by means of a temperature sensor additionally introduced into the heating system and keep the temperature constant by switching the power supply on and off, or they limit the supplied power by means of a timer control.
A special configuration of such a surface heating element consists of two opposed contact strips that generally consist of metallic conductors and have the function to supply electric power to the heating conductors which extend from contact strip to contact strip. This is accomplished in that the heating conductors intersect the contact conductors and are mechanically pressed against them, whereby an electrical contact is established. This creates a parallel connection of the heating conductors, the heating power being adjustable by selecting the electrical conductance values of the heating conductors.
Such surface heating elements are known from DE 4142774 A1 and DE 4020580 A1. According to DE 4142774 A1 the heating conductors run in a sinusoidal shape as weft threads over stitch threads of a knitted base material and, at least at the maxima of the amplitudes, are incorporated into the stitches of the textile base material. Power supply conductors are arranged at right angles to the sine axis as contact strips on mutually opposed pattern repeat edges, and each heating conductor is electrically conductively connected to each contact conductor. In one embodiment, a filament of carbon fibers can be used as the heating conductor. According to DE 4020580 A2 a planar electrical heating element consists of a net-type looped fabric incorporating heating conductors and contact conductors connected to the ends of the heating conductors. The heating conductors run essentially parallel to one another and are firmly bonded at intervals into the looped composite of the textile looped fabric in such a way that their path is loop- or wave-shaped or meandering. The contact conductors run essentially in a direction perpendicular to the heating conductors.
In this type of seat heater the contact strips must conduct a great amount of power relative to the heating conductors, so that these get overheated, which represents a drawback.
This problem is solved in the known surface heating elements in that the contact strips consist of copper or copper alloys, i.e. metals showing high conductance values.
A further problem associated with such seat heaters are the high mechanical loads arising in the seat, which may lead to breakage of parts of the contact strip or breakage of whole contact strips. Partial breakage of the contact strips results in cross-sectional size reductions and thus to lower conductance values on the contact strip in the area of the breakage point. As for the current, this leads to high power losses at the point of breakage and thus overheated locations.
To solve these problems, the conductors were given a wave-like layout, which is described in DE 4101290 C2. Use was also made of core/coat wires, the core of which consists of copper or a copper alloy, and the coating of which comprises steel, or vice versa. Such types of heating elements are known from DE 10206336 A1. Solutions with multiple feeds of the contact conductors are also known
All of these embodiments are still subject to the risk of breakage of the contact strip together with extreme wrinkling. Especially the technique to sew such a planar heating element into the seat cover entails the risk of wrinkling on the cover and heating element due to the vicinity of the heating element relative to the seat surface and thus also wrinkling and breakage of the contact strip, also in the above-indicated variants.